The reason for this interest is the relative ease to target secreted protein therapeutics into their place of action (body fluids or the cellular membrane). Secreted proteins, and the extracellular regions of transmembrane proteins, can be directly administered into body fluids, or can be directed to body fluids or membranes by a natural pathway. The natural pathway for protein secretion into extracellular space is the endoplasmic reticulum in eukaryotes and the inner membrane in prokaryotes (Palade, Science, 189, 347 (1975); Milstein, et al., Nature New Biol., 239, 117 (1972); Blobel, et al., J. Cell. Biol., 67, 835 (1975)). On the other hand, there is no known natural pathway for exporting a protein from the exterior of the cells into the cytosol (with the exception of pinocytosis, a mechanism of snake venom toxin intrusion into cells). Therefore, targeting protein therapeutics into cells poses extreme difficulties in the art.
IL-18 is a recently discovered novel cytokine. Active human IL-18 contains 157 amino acid residues. It has potent biological activities, including induction of interferon-γ-production by T cells and splenocytes, enhancement of the killing activity of NK cells and promotion of the differentiation of naive CD4+T cells into Th1 cells. In addition, human IL-18 augments the production of GM-CSF and decreases the production of IL-10. IL-18 has been shown to have greater interferon-γ inducing capabilities than IL-12, and appears to have different receptors and utilize a distinct signal transduction pathway.
CD4+ T cells are the central regulatory elements of all immune responses. They are divided into two subsets, Th1 and Th2. Each subset is defined by its ability to secrete different cytokines. Interestingly, the most potent inducers for the differentiation are cytokines themselves. The development of Th2 cells from naive precursors is induced by IL-4. Prior to the discovery of IL-18, IL-12 was thought of as the principal Th1 inducing cytokine. IL-18 is also a Th1 inducing cytokine and is more potent than IL-12 in stimulating the production of interferon-γ.
Th1 cells secrete IL-2, interferon-γ, and TNF-β. Interferon-γ, the signature Th1 cytokine, acts directly on macrophages to enhance their microbiocidal and phagocytic activities. As a result, the activated macrophages can efficiently destroy intracellular pathogens and tumor cells. The Th2 cells produce IL-4, IL-5, IL-6, IL-10 and IL-13, which act by helping B cells develop into antibody-producing cells. Taken together, Th1 cells are primarily responsible for cell-mediated immunity, while Th2 cells are responsible for humoral immunity.
Wound repair is a highly orchestrated interplay involving several cell types, extracellular matrix components, and multiple soluble mediators, including growth factors and cytokines. Wound repair may be triggered by trauma, microbes or chemicals, which have caused tissue injury. Although restoration of tissue integrity is an innate host immune response, there are situations during which the wound repair process is impaired. Several growth factors have been used to try to prevent mucositis in cancer patients undergoing radiation or chemotherapy with limited success. Peterson, Adv. Stud. Med., 4(4B): S299-S310, (2005). Granulocyte-colony stimulating factor (Neupogen) had a modest effect on the incidence and severity of mucositis in two out of four studies that involved cancer patients undergoing treatment. Granulocyte macrophage-colony stimulating factor (Sargramostim) induced a modest decrease in the severity of chemotherapy and radiation induced mucositis, although the results were inconsistent. Both granulocyte-colony stimulating factor and granulocyte macrophage-colony stimulating factor have only demonstrated an effect in the prevention of oral mucositis. Keratinocyte growth factor (Palifermin) has shown the most promise in the prevention of mucositis, preventing both the incidence and the duration of oral mucositis. With the emergence of agents that target mucositis pathophysiology, clinicians will no longer need to alter the radiation or chemotherapy regimens, but will tailor the protocol to include an agent that can prevent the incidence of mucositis. Clearly, there exists in the art a need to develop new therapeutic proteins to enhance wound repair, particularly to treat: skin wounds, surgical wounds, leg ulcers, diabetic ulcers, mucositis, particularly gastrointestinal mucositis and oral mucositis, and lung injury.